The invention relates to nanoporous aluminium oxide membranes, the nanopores of which contain ligand-stabilized gold clusters, a process for their production and their use for the production of red decorations.
Purple pigments based on colloidal gold or precursors thereof in and/or on a particulate carrier material, such as a glass frit or a metal oxide, and their use for the production of decorations on glass, porcelain and ceramic have been known for a long time, reference being made to DE-OS 44 11 104 and DE-OS 44 11 103 by way of example. The purple color can indeed be shifted somewhat further into the red region by co-using a small amount of a silver compound, but a pure red pigment based on colloidal gold or an intensely red decoration after application of a gold-containing pigment precursor onto a substrate which is stable to baking, with subsequent baking, has not been previously obtainable.
It has been possible to eliminate another disadvantage of gold-containing decoration preparations which are already known for the production of purple-colored decorations, that is to say an inadequate stability to baking at temperatures of about/above 1,000.degree. C., by using ligand-stabilized gold clusters in decoration preparations--see DE Patent Application 197 04 479.4. However, the purple color of the decoration was not changed, in comparison with other gold compounds, by using the gold cluster.
Nanoporous aluminium oxide membranes with hollow or solid gold fibrils in the pores are known--see C. R. Martin, Chem. Mater., vol. 8, no. 8 (1996), 1739-1746 and J. Phys. Chem. (1994), 98, 2963-2971. The gold can be deposited without a current or electrochemically. While the nanoporous Al.sub.2 O.sub.3 itself is optically transparent, the membrane assumes a color in the range from red-purple to blue because of the gold deposit. The color shifts from red-purple to blue as the length to diameter ratio of the fibrils increases; as the diameter of the fibrils decreases (150 nm to 20 nm), there is a shift towards red (in FIG. 6 of this document, this has obviously been transposed from top to bottom).
Nanoporous Al.sub.2 O.sub.3 membranes with spheroidal gold particles having a diameter of about 3 to 9 nm can be obtained by electrochemical deposition of gold in the pores of a nanoporous AL.sub.2 O.sub.3 membrane obtained by anodic oxidation in phosphoric acid solution--see J. Preston et al., J. Phys. Chem. (1993), 97, 8495-8503. A cluster structure has been assigned to the gold particles of the size mentioned; the gold content was about 0.1 to 2 wt. %. There is no suggestion of using nanoporous Al.sub.2 O.sub.3 membranes containing gold compounds or elemental gold particles as agents for production of red decorations on substrates which are stable to baking in the documents acknowledged above.